Leading climate scientist Kevin Trenberth has toldreportingclimatescience.com that he believes the pause in global warming may be caused by long term changes in the Pacific Ocean.

Trenberth and colleague John Fasullo argue in a new scientific paper that the massive El Nino Pacific Ocean warming event that occurred in 1997 and 1998 triggered the pause. They say that the El Nino caused a large loss of heat from the deep ocean to the sea surface that resulted in a cooling of the oceans. Since then the deep ocean has been absorbing heat back from the upper ocean and so cooling the atmosphere.

The implication is that the heat being absorbed from the atmosphere by the oceans has offset the underlying and ongoing warming of the atmosphere due to green house gases. As the deep ocean waters have slowly warmed they have taken heat from the upper ocean which has then cooled the atmosphere. This is the cause of the apparent hiatus in global warming that has manifested itself as a halt in the rise in global mean atmospheric temperatures seen in the second half of the 20th century.

Trenberth and Fasullo, from the US National Center for Atmospheric Research in Boulder Colorado, suggest that long term oscillations in the Pacific Ocean, known as Pacific Decadal Oscillations (or PDOs) drive alternate 20-plus year cycles of upper ocean warming and cooling which also involve heat being exchanged with the atmosphere. The implication of this is that when the Pacific is in a negative phase the upper ocean loses heat and so cools the atmosphere, and that when it is in a positive phase the upper ocean warms and so heats the atmosphere.

“It is not so much that the atmosphere warms up rather that the upper levels of sea get warmer and these interact more directly with the atmosphere,” Trenberth said. So a warmer sea surface leads to a warmer atmosphere. “More heat penetrates to the deep ocean in the negative phase and that is not the case in the positive phase,” he explained.

“We can speculate that the huge 1997–1998 El Niño event was a trigger for the change in the PDO; certainly, it led to a large loss of heat in the Pacific… that has taken years to recover from, if the recovery is even complete. Past behavior of the PDO… suggests that regimes can last for 25 years,” Trenberth and Fasullo write in their paper.

“The picture emerging is one where the positive phase of the PDO from 1976 to 1998 enhanced the surface warming somewhat by reducing the amount of heat sequestered by the deep ocean, while the negative phase of the PDO is one where more heat gets deposited at greater depths, contributing to the overall warming of the oceans but cooling the surface somewhat. The Pacific Ocean appears to account for the majority of the decadal variability… Nevertheless, the events in the Pacific undoubtedly also affect the Atlantic, Indian, and Southern Oceans as the system acts collectively to equilibrate to these changes in the flow of energy,” they write.

“There are really deep teleconnections between the Pacific and the Atlantic and Southern Oceans,” Trenberth explained. “The centre of action is the Pacific Ocean but the main places where heat goes deep into the ocean are the Atlantic and Southern Oceans rather than the Pacific.”

There is also a very strong relationship with winds and sea level, according to Trenberth. Water is piling up in the western Pacific Ocean at a rate of around 10mm per year which is three times the global average. This has led to a difference in sea level, measured by satellite radars, between the western and eastern Pacific. “The sea level is 20cm higher in the western Pacific and the only way to keep it there is for strong winds to pile up the water. It is these changes in the winds that change the ocean currents and affect where the heat is going,” he explained. “But this can’t keep going for ever. The ocean wants to slop back to the east.”

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